TOKYO - An investigation is under way in Japan after a Maritime Self-Defense Force destroyer rammed a small fishing boat in the Pacific, cutting the trawler in half.

The father-and-son fishermen who were in the boat were still missing Wednesday despite a massive search effort that included a dozen ships and four helicopters as well as a team of divers, The Japan Times reported.

The top MSDF commander, Adm. Eiji Yoshikawa, told the newspaper the sea was calm and visibility was good at the time of the incident. He said the destroyer Atago had apparently spotted the fishing boat because it tried to avoid the collision.

Tuesday's incident was the first serious one involving an MSDF vessel since the submarine Nadashio collided with a charter fishing boat in Tokyo Bay in 1988, killing 30 people and injuring 17 others.